Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, application specific integrated circuits (ASICs), and other specific functional circuits.
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, creating visual projections for television displays, and buffering transmission lines. Semiconductor devices are found in the fields of entertainment, communications, power conversion, mechanical control, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
In semiconductor devices, it is often necessary to transmit clock signals or data signals over a distance, e.g. micrometers (μm) to meters. To maintain the noise margin and integrity of the signal, a buffer is used to drive the signal along a transmission line. FIG. 1 shows a conventional current mode logic (CML) buffer circuit 10 receiving a differential clock signal or data signal at terminals 12 and 14. The CLK signal is applied to the gates of n-channel metal oxide semiconductor field effect transistors (nMOSFET) 16 and 18. The sources of transistors 16 and 18 are coupled to current source 20 referenced to a low operating potential (ground) at terminal 22. The drains of transistors 16 and 18 are coupled to resistors 24 and 26 as resistive loads. Resistors 24 and 26 are coupled to inductor 27, which is center tapped to a high operating potential, e.g., VDD=+2 to +5 VDC, at terminal 28.
For a p-type substrate, a “high or higher voltage” generally refers to a positive voltage less than or equal to the maximum positive operating potential (VDD) and greater than a “low or lower voltage”, and the “low or lower voltage” generally refers to a positive voltage greater than or equal to the minimum operating potential (ground) and less than the “high or higher voltage.” When the CLK signal at terminal 12 is high with respect to terminal 14, transistor 16 switches to a conductive state (VGS16 greater than threshold VTH of the transistor) and pulls the voltage at node 30 to a low value. Transistor 18 is non-conductive (VGS18 not greater than VTH) and resistor 26 pulls the voltage at node 32 to a high value. When the CLK signal at terminal 14 is high with respect to terminal 12, transistor 18 switches to a conductive state (VGS18 greater than VTH) and pulls the voltage at node 32 to a low value. Transistor 16 is non-conductive (VGS16 not greater than VTH) and resistor 24 pulls the voltage at node 30 to a high value.
The combination of transistors 16 and 18 and current source 20 drives the voltages at nodes 30 and 32 along transmission line 34. Transmission line 34 can be shielded and grounded, ranging from μm to meters in length. A terminating end of transmission line 34 is coupled to the gates of nMOSFETs 36 and 38, respectively. A pair of 50 ohm pull-up resistors 40 and 42 are coupled between the terminating end of transmission line 34 and terminal 28 operating at VDD. The sources of transistors 36 and 38 are coupled to current source 44 referenced to a low operating potential (ground) at terminal 22. The drains of transistors 36 and 38 are coupled to resistors 46 and 48 as resistive loads. Resistors 46 and 48 are coupled to inductor 49, which is center tapped to a high operating potential, e.g., VDD=+2 to +5 VDC, at terminal 28.
When the signal from transmission line 34 at the gate of transistor 36 is high with respect to the signal from transmission line 34 at the gate of transistor 38, transistor 36 switches to a conductive state (VGS36 greater than VTH) and pulls the voltage at output terminal 50 to a low value. Transistor 38 is non-conductive (VGS38 not greater than VTH) and resistor 48 pulls the voltage at output terminal 52 to a high value. When the signal at the gate of transistor 38 is high with respect to the signal at the gate of transistor 36, transistor 38 switches to a conductive state (VGS38 greater than VTH) and pulls the voltage at output terminal 52 to a low value. Transistor 36 is non-conductive (VGS36 not greater than VTH) and resistor 46 pulls the voltage at output terminal 50 to a high value.
CML buffer circuit 10 is a broadband amplifier using resistive drain loads for long distances. When driving high-speed clocks along long transmission lines, there is a trade-off between the distances that can be driven at a particular clock-frequency versus power. CML buffer circuit 10 tends to consume significant power, which is a disadvantage in minimal power configurations, such as battery applications.